Specialty control thermocouple for vacuum heat treat furnaces

Abstract

A low thermal conductivity fixed thermocouple with a heat sink package specifically designed for electrically heated vacuum furnaces having an overall cylindrical hot zone diameter of 36 inches or less, and preferably containing all metal reflective radiation shields or graphite felt insulation packages that experience high conductive losses in the low temperature ranges during vacuum heating which result in large discrepancies between the furnace temperature readings and the actual workload temperature.

What is claimed is: 
     
       1. In a cold wall vacuum furnace hot zone with a heating element diameter of 36 inches or less and operating at temperatures of 649″C or less for heat treating metal parts, a control thermocouple assembly for measuring the temperature in the hot zone comprising:
 continuous sheath having an elongated longitudinal axis and a measuring tip located along said elongated longitudinal axis at a distal end of said sheath; 
 said sheath having an outside diameter of no more than 3/16 inch and being made from a ceramic material; 
 said sheath having an inner single bore ceramic tube comprised of a series of stacked tubes having air gaps therebetween located one on top of another within said sheath, 
 said sheath containing a first thermocouple wire therein and the series of stacked tubes having a single bore containing a second thermocouple wire therein, said first and second thermocouple wires being separated from each other within said sheath, and being joined together at the longitudinal tip of said sheath to form a thermocouple junction therebetween; and 
 a reflection shield being operatively connected to an outer face of the furnace hot zone and located in proximity to said thermocouple assembly sheath. 
 
     
     
       2. The control thermocouple assembly according to  claim 1  wherein said ceramic material of said sheath is alumina. 
     
     
       3. The control thermocouple assembly according to  claim 1  wherein said ceramic material of said sheath is made of any high temperature ceramic material. 
     
     
       4. The control thermocouple assembly according to  claim 1  wherein said reflection shield comprises a series of reflection shields connected together within a housing to form a single unit, said housing being operatively connected to the outer face of the furnace hot zone. 
     
     
       5. The control thermocouple assembly according to  claim 4  wherein said reflection shields are made of molybdenum. 
     
     
       6. The control thermocouple assembly according to  claim 4  wherein said reflection shields are made of stainless steel. 
     
     
       7. The control thermocouple assembly according to  claim 4  wherein said reflection shields are approximately 0.015 inches thick. 
     
     
       8. The control thermocouple assembly according to  claim 4  wherein each of said reflection shields are separated by approximately 1/16 inch or greater. 
     
     
       9. The control thermocouple assembly according to  claim 1  wherein said first thermocouple wire is made from platinum 24 gauge (Type S). 
     
     
       10. The control thermocouple assembly according to  claim 1  wherein said second thermocouple wire is made from platinum/rhodium (10%) 24 gauge (Type S). 
     
     
       11. The control thermocouple assembly according to  claim 1  wherein said first and second thermocouple wires are made from any material similar to platinum 24 gauge or platinum/rhodium (10%) 24 gauge (Type R).